1. Field of the Invention
The invention is concerned with devices entailing the reflection of electromagnetic radiation--specifically in which the reflection is by a principle sometimes referred to as Distributed Bragg Reflection. Reflectors of this nature serve a variety of uses and all are of consequence to the invention. Uses include simple reflection in which the device contemplated may be the reflector itself as well as more sophisticated devices, e.g. including cavities, in which cavitation, of coherent or incoherent light is at least in part attributed to Distributed Bragg Reflection.
2. Description of the Prior Art
Distributed Bragg Reflectors (DBR) are of increasing significance due largely to their capability of more completely reflecting energy. This advantage is retained for essentially the entire spectrum of electromagnetic energy although specific design is for a specific fractional spectrum. Whereas simple or single interface reflectors continue to serve their many purposes, effectiveness is wavelength-dependent. For many uses resort must be had to DBR e.g. as in the x-ray spectrum. At other wavelengths, so-called "effective reflection" of perhaps as great as 95% or even 98+% is inadequate or at least awkward. Laser cavities designed both for effective emission and e.g. to take heat dissipation into account, may take advantage of 99+% cavity reflection--a value not generally realizable for simple reflection.
DBR design and fabrication is dependent upon a number of considerations--some economic, some functional. Depending as they do upon accumulated reflection resulting from successive contributions at interfaces between successive pairs of material of differing refractive index, operation is dependent upon the refection/transmission ratio at concerned interfaces--a characteristic determined by .DELTA.n (the difference of refractive index of the two materials forming the interface). This value, in turn, determines the number of layers required for a given reflectivity. Increasing .DELTA.n has economic implications in permitting a smaller number of layers, has yield implications in that fabrication simplification (attendant e.g. on need to deposit lesser number of layers) statistically lessens flaws, and has operational implications e.g. in that reflection traversal path may be shortened, thereby improving response time.
Most demanding DBR structures, for example, those used in laser cavitation, depend upon a very high degree of crystalline perfection. This need is satisfied by a variety of epitaxial growth techniques. Procedures finding acceptance in laser fabrication at this time include Molecular Beam Epitaxy, Chemical Vapor Deposition and variants. All such procedures as applied, for example, to growth of layers of permitted thickness variation corresponding with a monolayer as referenced to nominal layer dimensions of hundreds of .ANG., are extremely time consuming and expensive. Vertical Cavity Surface Emitting Laser (VCSEL) cavities, commonly requiring very high reflectance efficiency, may entail several hours of deposition processing in expensive, high space consuming equipment. Some have estimated that such processing represents 50% or more of device cost.
Operationally, DBR complexity has consequences of significance. Dependence upon many-layered reflectivity statistically increases interfacial defects, dimensional inhomogeneity--e.g., layer thickness variation, as well as increasing cavity path length. It is to be expected that all such considerations are favorably affected by decreasing the number of Bragg layers. Since this design criterion--the number of layers required for a desired value of reflectivity--varies with the ratio of indices of refraction at the interface, every effort is made to seek out materials of largest .DELTA.n in otherwise satisfactory materials (e.g., materials of desired atomic dimension, morphology, crystalline orientation).
While the search for still more effective materials continues, the need is less for certain wavelength regimes. For example, available .DELTA.n values for 99.7% cavity reflectivity at wavelengths .lambda.=0.85-0.98 .mu.m is sufficient to realize laser cavitation for a single quantum well GaAs-based VCSEL using DBR mirrors of.apprxeq.18 pairs of GaAs/AlAs (.DELTA.n=.apprxeq.0.65). Cavitation for other wavelengths is not so easily attained. For example, InP-based materials used in laser cavitation at.apprxeq.1.55 .mu.m have not offered such .DELTA.n values. A single quantum well VCSEL for operation at this wavelength has not yet been publicity demonstrated and would likely require as many as 40 Bragg pairs for each mirror (.DELTA.n=.apprxeq.0.3).
DBR cavities, essential to the operation of (coherent) laser devices described, serve a secondary function which may be of value for incoherent emission--e.g., in providing directionally with implicit advantage in terms of brightness for efficiency (brightness of the desired field of view). In general, cavitation of such incoherent sources has been impeded by unavailability of mirrors accommodating the spectral emission range which is quite broad relative to that of a laser. This impediment would be alleviated by large values of .DELTA.n. (Wavelength dependence of reflectivity decreases as .DELTA.n increases.) A number of other uses will be well served by .DELTA.n values greater than those presently available. Such uses include wavelength sensitive detectors, optical logic etalons, optical modulators, as well as systems and processes requiring high reflectivity mirror elements. The latter category includes various types of projection, and, at this time, may be of consequence in projection lithography e.g. for fabrication of devices of submicron dimensions.